PROJECT SUMMARY Myelodysplasia (MDS), an acquired clonal disease of the hematopoietic stem cell (HSC), is on the rise in the aging population with poor overall survival. Mutations in key factors of the spliceosome have been identified in over 50% of patients and mutations in SRSF2 are the most frequent mutations identified. How mutations in SRSF2 contribute to MDS is not known. Mutations in SRSF2 uniquely affect proline at position 95, within the C-terminus of the RNA binding domain. We have recently shown that mutations of P95 to histidine (P95H), leucine (P95L), or arginine (P95R) alter the structure of the SRSF2 RRM, resulting in altered RNA binding affinity and specificity, thereby leading to aberrant splicing. Our aim is to further understand how mutations in SRSF2 mutations affect RNA binding in vivo and how altered splicing affects hematopoietic stem cell maintenance and differentiation. Specifically, we seek to 1) understand how mutations in SRSF2 disrupt its function in vivo, 2) determine the pathways disrupted by alternative splicing at the root of MDS pathology, and 3) develop therapeutic approaches for SRSF2 mutant MDS. We will determine RNA targets and RNA target motifs in vivo using RNA immunoprecipitation in conjunction with UV crosslinking and high throughput sequencing (HITS-CLIP) as well as RNAseq to determine how SRSF2 mutations affect its function in vivo and identify essential targets affected by alternative splicing. We will determine the function of splice isoforms of critical down-stream targets, in particular of other RNA binding proteins and splicing factors that are alternatively bound and spliced by mutant SRSF2. We will identify alternative splice events critical to stem cell maintenance and hematopoietic progenitor proliferation and differentiation and determine how SRSF2 mutations alter their stage and lineage specific occurrence. Based on our structure-function studies we have rationally designed first-generation small molecules that target SRSF2, and we will further develop these compounds for therapeutic purposes of SRSF2 mutant MDS. Our combined molecular and biologic studies, small molecule design, and in vivo approaches promise to greatly advance understanding and treatment of SRSF2 mutant MDS.